1. Technical Field
The present invention relates to a method of fabricating a nitride based semiconductor substrate for use in a nitride based semiconductor laser or the like which is expected to be applied to fields such as optical information processing and radio communication, and a method of fabricating a nitride based semiconductor device.
2. Related Art
A nitride based semiconductor having nitride (N) as V-group element is considered to be promising as a material of a short wavelength light-emitting element and a high power semiconductor circuit because of its large bandgap. In particular, a gallium nitride based compound semiconductor (GaN based semiconductor: AlxGayInzN(0xe2x89xa6x,y,zxe2x89xa61, x+y+z=1)) has been intensively studied, and a blue light-emitting diode (LED) and a green LED have been put into practical use. Meanwhile, for achieving a large-capacity optical disc system, a semiconductor laser having an oscillation wavelength in 400 nm band has come to draw attention. At present, the semiconductor laser is practically used.
FIG. 1 is a cross-sectional view schematically showing a structure of the conventional GaN based semiconductor laser. As shown in FIG. 1, on a sapphire base 1701, a GaN buffer layer 1702, a n-GaN layer 1703, a n-AlGaN cladding layer 1704, a n-GaN light guiding layer 1705, a multiple quantum well (MQW) active layer 1706 comprised of Ga1-xInxN/Ga1-yInyN (0 less than y less than x less than 1), a p-GaN light guiding layer 1707, a p-AlGaN cladding layer 1708, a p-GaN contact layer 1709 are deposited as crystals grown by a metalorganic Vapor Phase Epitaxy (MOVPE) process. On the p-GaN contact layer 1709, a ridge strip having a width of approximately 3 xcexcm is provided, and both sides thereof are covered by the insulating film such as SiO2 1711. On the ridge strip and the SiO2 1711, a p electrode 1710 comprised of, for example, Ni/Au, is provided, and an electrode 1712 comprised of, for example, Ti/Al is provided on a surface of part of the n-Ga N layer 1703 exposed by etching.
In the semiconductor laser so structured, upon the n electrode 1712 being grounded and a forward voltage being applied to the p electrode 1710, positive holes migrate from the p electrode 1710 side toward the MQW active layer 1706 and electrons migrate from the n electrode 1712 side toward the MQW active layer 1706. This results in optical gain inside the MQW active layer 1706 and laser oscillation having an oscillation wavelength of 400 nm band. The oscillation wavelength varies depending on a composition and thickness of Ga1-xInxN/Ga1-yInyN thin film as a material of the MQW active layer 1706. At present, continuous oscillation at temperatures higher than a room temperature, is implemented. A high-power semiconductor circuit using these techniques is studied and is expected to be achieved in fields such as semiconductor devices for radio communications.
As a substrate on which GaN based crystal is grown, a sapphire base, a SiC (silicon carbide) substrate, or a Si (silicon) substrate is used. But, these substrates lattice-mismatch to GaN, and therefore crystal growth becomes difficult. This results in a number of dislocations (blade-shaped dislocation, spiral dislocation, mixed dislocation). For example, when using the sapphire base or the SiC substrate, dislocations of approximately 1xc3x97109 cmxe2x88x922 exist. As a result, a threshold current of a semiconductor laser is increased and reliability of the semiconductor laser is degraded.
A first article as a known literature xe2x80x9cJournal of Material Research, Vol. 14 (1999) pp. 2716-2731xe2x80x9d proposes an Epitaxial Lateral Over Growth (ELOG) as a method of reducing dislocation density. This method is effective in reducing through dislocations in a system having large lattice mismatching.
FIG. 2 is a cross-sectional view schematically showing a structure of GaN crystal formed by ELOG. On a sapphire base 1801, GaN crystal 1802 is formed by the MOVPE process or the like. On the GaN crystal 1802, SiO2 1803 is formed by a CVD (Chemical Vapor Deposition) process or the like. The SiO2 1803 is processed in stripes by photolithography and etching. A GaN based semiconductor layer 1804 is deposited by selectively growing an exposed portion of the GaN crystal 1802 as seed crystal. As a growing process, the MOVPE process or a hydride vapor phase epitaxy process (HVPE process) is used. Above the seed crystal, a region 1806 having a number of dislocations of approximately as high as 1xc3x97109 cmxe2x88x922 exists, but a region 1805 which is laterally grown has dislocation density as low as approximately 1xc3x97107 cmxe2x88x922. An active region is provided above the region 1805 with fewer dislocations, thereby improving reliability. Since the other structure in FIG. 2 is identical to a structure of the conventional semiconductor laser in FIG. 1, the same or corresponding parts are identified by the same reference numerals and will not be further described.
In recent years, fabrication of a GaN substrate has been intensively studied.
A second article as a known literature xe2x80x9cJapanese Journal of Applied Physics, Vol. 37 (1998) pp. L 309-L312xe2x80x9d illustrates a method in which a sapphire base is removed by polishing in a GaN based semiconductor layer grown on the sapphire base, thereby obtaining a GaN substrate. A third article as a known literature xe2x80x9cJapanese Journal of Applied Physics, Vol. 38 (1999) pp. L217-L219xe2x80x9d illustrates a method in which a GaN based semiconductor layer is separated (lifted off) from the vicinity of a sapphire base by irradiation of a laser beam using a third harmonic (wavelength of 355 nm) of Nd:YAG laser. It is considered that the GaN based semiconductor layer is thus separated by irradiation of the laser beam due to the fact that the GaN based semiconductor layer in the vicinity of the sapphire base has low quality and high carrier concentration.
Related Arts are disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 11-191657 that discloses a method of growing nitride semiconductor and Japanese Laid-Open Patent Application Publication No. 2001-93837 that discloses a structure of a semiconductor thin film and a fabrication method thereof.
However, in the methods illustrated in the second and third articles, due to difference in thermal expansion coefficient between sapphire and GaN, a number of cracks occur in the GaN based semiconductor layer when separating the GaN based semiconductor layer from the sapphire base. For this reason, a GaN substrate having a large area equal to a two-inch wafer level is impossible to obtain. In addition, in these methods, it is not easy to control separation between the sapphire base and the GaN based semiconductor layer.
In a semiconductor device with the GaN based semiconductor layer disposed on the sapphire base, the GaN based semiconductor layer is subjected to a stress due to large difference in lattice constant between the sapphire base and the GaN based semiconductor layer grown thereon. This reduces reliability of yield and productivity as well as an electric property. Therefore, it is necessary to separate the GaN based semiconductor layer from the sapphire base and form constituents on the GaN based semiconductor substrate.
The present invention has been made under the circumstances, and an object of the present invention is to provide a method of fabricating a nitride based semiconductor substrate with high controllability in separation between the sapphire base and the GaN based semiconductor layer.
To achieve the above-described object, according to the present invention, there is provided a method of fabricating a nitride based semiconductor substrate, comprising the steps of depositing a first nitride based semiconductor layer on a base; processing the first nitride based semiconductor layer to have ridge portions and recess portions; coating side surfaces of the ridge portions and bottom surfaces of the recess portions with an amorphous insulating film; growing a second nitride based semiconductor layer on a region of the first nitride based semiconductor layer other than a region thereof coated with the amorphous insulating film, the region of the first nitride based semiconductor layer serving as seed crystal; and separating the second nitride based semiconductor layer from the ridge portions by irradiating the region corresponding to the seed crystal with a laser beam.
It is preferable that the method of fabricating a nitride based semiconductor substrate, further comprises the step of thermally annealing the base with the first and second nitride based semiconductor layers deposited thereon, before the separating step.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the laser beam has a wavelength of 190 nm to 550 nm.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the ridge portions are ridge stripes and a direction of the stripes is a  less than 1-100 greater than  direction of nitride.
According to the present invention, there is provided a method of fabricating a nitride based semiconductor device, comprising the steps of depositing a first nitride based semiconductor layer on a base; processing the first nitride based semiconductor layer to have ridge portions and recess portions; coating side surfaces of the ridge portions and bottom surfaces of the recess portions with an amorphous insulating film; growing a second nitride based semiconductor layer on a region of the first nitride based semiconductor layer other than a region thereof coated with the amorphous insulating film, the region of the first nitride based semiconductor layer serving as seed crystal; depositing a layer having an active layer structure with an active layer interposed between semiconductor layers of different conductivity types, on the second nitride based semiconductor layer; and separating the second nitride based semiconductor layer from the ridge portions by irradiating the region corresponding to the seed crystal with a laser beam.
According to the present invention, there is further provided a method of fabricating a nitride based semiconductor substrate, comprising the steps of depositing a first nitride based semiconductor layer on a base; depositing a second nitride based semiconductor layer on the first nitride based semiconductor layer; processing the first and second nitride based semiconductor layers to have ridge portions and recess portions; coating side surfaces of the ridge portions and bottom surfaces of the recess portions with an amorphous insulating film; growing a third nitride based semiconductor layer on a region of the second nitride based semiconductor layer other than a region thereof coated with the amorphous insulating film, the region of the second nitride based semiconductor layer serving as seed crystal; and separating the second nitride based semiconductor layer from the ridge portions by irradiating the region corresponding to the seed crystal with a laser beam.
It is preferable that the method of fabricating a nitride based semiconductor substrate, further comprises the step of depositing a layer having an active layer structure with an active layer interposed between semiconductor layers of different conductivity types, on the third nitride based semiconductor layer, before the separating step.
It is preferable that the method of fabricating a nitride based semiconductor substrate may further comprise thermally annealing the base with the first, second, and third nitride based semiconductor layers deposited thereon.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the ridge portions are ridge stripes and a direction of the stripes is a  less than 1-100 greater than  direction of nitride.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the laser beam has a wavelength of 190 nm to 550 nm.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the second nitride based semiconductor layer is comprised of semiconductor compound containing three or more elements belonging to III-V groups.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the second nitride based semiconductor layer has bandgap smaller than bandgap of the third nitride based semiconductor layer.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the second nitride based semiconductor layer contains at least As.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the second nitride based semiconductor layer contains at least In.
It is preferable that in the method of fabricating a nitride based semiconductor substrate, the second nitride based semiconductor layer contains at least P.
According to the present invention, there is further provided a method of fabricating a nitride based semiconductor device, comprising the steps of depositing a first nitride based semiconductor layer on a base; depositing a second nitride based semiconductor layer on the first nitride based semiconductor layer; processing the first and second nitride based semiconductor layers to have ridge portions and recess portions; coating side surfaces of the ridge portions and bottom surfaces of the recess portions with an amorphous insulating film; growing a third nitride based semiconductor layer on a region of the second nitride based semiconductor layer other than a region thereof coated with the amorphous insulating film, the region of the second nitride based semiconductor layer serving as seed crystal; depositing a layer having an active layer structure with an active layer interposed between semiconductor layers of different conductivity types, on the third nitride based semiconductor layer; and separating the second nitride based semiconductor layer from the ridge portions by irradiating the region corresponding to the seed crystal with a laser beam.
This object, as well as other objects, features and advantages of the invention will become more apparent to those skilled in the art from the following description taken with reference to the accompanying drawings.